1. Technical Field
This disclosure relates to a cathode material structure for a lithium-ion battery, and a method for preparing the cathode material structure.
2. Related Art
Current portable electronic products, such as digital cameras, cellular phones and notebook computers, require lightweight batteries. Among various types of batteries, the rechargeable lithium-ion battery has a unit-weight capacity three times higher that of the traditional lead storage battery, nickel-metal-hydride battery, Ni—Zn battery or Ni—Cd battery etc. and can be rapidly charged, and therefore has been used widely.
FIG. 1 schematically depicts a conventional lithium-ion battery, which includes a cathode 100, an anode 110, and an electrolyte 120 and a separator 130 between the two. The cathode 100 includes a cathode metal 102, and a cathode material 104 fixed/coated on the cathode metal 102 via a binder mixed with a conductive material. The anode 110 includes an anode metal 112 and an anode material 114 thereon.
The cathode material of a lithium-ion battery is usually an oxide of lithium and other metal(s), which has the following issues. When an internal short circuit occurs to release much heat, the cathode material tends to be damaged in the crystal structure to decompose, and may even release oxygen gas that reacts with the organic electrolyte to cause fire or even explosion. A current solution is to use a cathode material with high thermal/chemical stability, such as LiMn2O4, LiFePO4, LiNi1/3CO1/3Mn1/3O2 (LNCM), Li4Ti5O12 (LTO) or a combination thereof, which is not easily damaged in the crystal structure and does not easily generate oxygen gas, and is therefore safer. However, the capacity, power density or working voltage of the battery is often reduced by doing so.
Moreover, the continuous growth of the solid electrolyte interface (SEI) film will irreversibly raise the internal resistance of the battery, so that the capacity, power density, cycle lifetime, working voltage and charge/discharge efficiency are lowered rapidly.
Moreover, the overcharge resistance and over-discharge resistance of the cathode material also have to be raised. That is, in an overcharge or over-discharge operation, the cathode material on the cathode is required to exhibit no remarkable expansion, extension or shrinkage and also to prevent partial decomposition and deposition of the lithium component on the anode.
In addition, the internal resistance of current cathode material is high, so that the specific energy is lowered rapidly at high power output. The mechanical performances and tenacity of the cathode material structure also have to be enhanced to prevent crack or damage due to external compression, especially to prevent falling-off of the active material that would make a direct electrical connection to cause an internal short circuit. The chemical stability of the cathode material is also important, for the cathode material cannot be dissolved in the electrolyte and must have stable redox reactions.
Furthermore, when the lithium-ion battery is used at high temperature and low temperature repeatedly, the irreversibility of the cathode material is gradually increased so that the battery performance is gradually lowered with time. Although raising the charging termination voltage can increase the discharge capacity (specific energy) of the lithium-ion battery, the lifetime of the battery is shortened by doing so.